Chromium stainless steels



Patented Nov. 28, 1950 CHROMIUM STAINLESS STEELS Mary Baeyertz, ChicaLindsey,

go, 111., and Willard M. Rumford, R. I., assignors to Carnegie- IllinoisSteel Corporation, a corporation of New Jersey N Drawing. ApplicationJune 21, 1946, Serial No. 678,361

1 Claim. 1

This invention relates to chromium stainless steels and, particularly,to chromium stainless steels which are subjected to repeated heating andpickling when being formed to make certain shapes or articles possessexceptional stability to localized corrosive attack induced by acombination of heating followed by application of corroding media.

Steels of the aforementioned class containing, substantially, 5 to 14per cent chromium, and 0.05 to 0.25 per cent carbon, are widely used inindustry for manufacturing articles intended to possess reasonably highmechanical properties and good creep-resistant characteristicsassociated with a marked corrosion resistance. Industrial considerationsadvance forging as the preferred method of shaping thereof, and hotforming under a hammer or in dies occupies at present the foremostposition among methods used therefor.

Frequent intricate configuration of such forgings often calls forseveral forging operations. Since closeness of forging dimensions isusually imperative, the industry adopted the practice of removing scaleformed on forgings prior to sub- ,iecting them to a subsequent forgingstep. Pickling in inorganic acids or solutions thereof may be held asthe preferred method for achieving aforementioned scale removal, and iswidely used in forging practice.

Extensive industrial experience demonstrated that steels of the classdescribed do not respond to said pickling in an identical manner. Someof said steels soaked for one hour at 1950 F., forged, air cooled andpickled in a 50 per cent H01 solution at 170 F. until the scale isremoved, develop uniform clean surface. Identical sequence of the samesteps produces on the surface of other steels, still fully within thecompositional range of aforementioned alloys, a plurality of disorientedsurface discontinuities having macroscopic dimensions and a shape whichjustified attributing thereto the term wrigglers in the plant parlance.The surface appearance thereby produced closely resembles the surface ofburnt steel forgings fully familiar to those skil ed in the art.

We have found the explanation thereof in a critical relation betweenchromium and carbon content. The ranges involved therein are quitenarrow and, when the steel must meet certain mechanical-propertiesspecifications while remaining free from wrigglers, may render steelmanufacture extremely diflicult.

For the purpose of illustration, there may be used a case where desiredproperties require the composition of 0.0? to 0.12 per cent carbon,11.50 to 12.50 per cent chromium and 0.40 to 0.60 per cent molybdenum,other elements being present in the amounts common to the grade, andfreedom from aforementioned surface disconinuities is specified. Ourinvestigations demonstrated that utilization of the whole range ofcarbon and chromium inevitably leads to the failure of meeting therequirements with regard to immunity to the surface discontinuitiesknown as wrigglers. These are prone to occur when the carbon fallswithin the upper limits of the range or the chromium falls in the lowerpart of specifled range.

The requirements can be met in the above case only when the chromiumrange is restricted to 12.0 to 12.5 per cent, and carbon is limited to0.07 to 0.09 per cent. It is easily seen that working a heat withinthree points of carbon commands an exceptionally close control ofoperating steps, and the diiiiculties involved therein increase thepercentage of rejected heats to the point prohibitive of commercialoperation.

We have found that the ratio between the permissible carbon content andthe percentage of chromium inducive to the desired freedom from surfacediscontinuities remains substantially the same with the changingpercentage of chromium. Larger amounts of the latter element present insteel permit the use of a higher carbon content. However, the narrowrange of substantially three points of carbon for the variations ofchromium encompassed by one-half of one per cent thereof remains aprerequisite for an adequate performance of the steel, with allinconveniences inherent thereto.

We have found that the necessity for such exceptionally close control ofcarbon is eliminated by addition to the steel of a certain criticalamount of elements belonging to the group: titanium, columbium, andboron, without any detrimental eiiect on the physical orcorrosionresistant characteristics of the metal.

Permissible and desirable range of concentrations of the aforementionedelements is specified by two limits, namely, the presence of asufiicient minimum for inhibition of surface discontinuitiesdevelopment, and the avoidance of the percentage thereof inducive tochange in the response of steel to normal heat treating operations. Thussuch elements should not be present in such concentration that asubstantial amount of austenite is retained on COOIiIlg after forging.

The lower concentration limit of said additional elements depends onboth carbon and chromium content of steel. For steels containing, for example, 11.0 to 12.5 per cent chromium and 0.08 to 0.11 per cent carbon,a minimum columbium content of substantially 0.25 per cent, titanium ofsubstantially 0.08 per cent, and boron of 0.003 per cent have been foundadequate for the elimination of surface discontinuities after repeatedheating and pickling, at least after seven repetitions of the cycle. 7

Our studies of the properties of steel having the aforementionedexemplary composition as a function of increased percentages oftitanium, columbium, and boron alloyed with said steel, indicated thatthe properties undesirable from the standpoint of the application ofsaid steel to industrial uses begin to develop when columbium contentexceeds substantially 1.00 per cent, titanium percentage reachessubstantially 0.50 per cent and boron concentration becomes greater thanabout .05 per cent.

Steels embraced within the scope of the present invention include alloysgenerally known as chromium stainless steels and which comprise, besidesthe principal constituents, namely iron, chromium, and carbon, a certainpercentage of manganese, silicon, molybdenum, nickel and other elementspresent in the concentrations insuflicient, in the combination thereof,for transforming said steels to stable austenitic alloys.

Other modifications of applying the principles of our invention may beemployed within the scope of the appended claim.

We claim:

The method of producing forged articles posed. .of 11 to 12.5% chromiumsteel free from surface discontinuities comprising adding between .003and 05% boron to straight chromium steel containing between 11 and 12.5%chromium and 0.8 to 11% carbon, said boron addition being 4 regulated toprevent the formation of surface discontinuities on said steel resultingfrom acid pickling to remove scale resulting from heating to forgingtemperatures, forging articles from blanks of said steel, said forgingcomprising repeatedly heating said blanks to forging temperature,forging at said temperature, cooling and acid pickling the sameintermediate each heating to remove the scale formed by heating toforging temperature.

MARY BAEYER'IZ.

WILLARD M. LINDSEY.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,519,388 Walter Dec. 16, 19242,110,891 Reitz et al Mar. 15, 1933 PATENTS Number Country Date 375,793Great Britain June 20, 1932 OTHER REFERENCES The Book of StainlessSteels, 2nd Edition, Edited'by Ernest Thum; pages 264 and 268. Publishedin 1935 by the American Society for Metals, Cleveland, Ohio.

Stainless Iron and Steel, pages 59 and 852. Edited by Monypenny.Published in 1931 by Chapman-Hall, London, England.

Advantages of Columbium in Wrought 4 to 6 Chromium Steel, pages 10 and16. Published in 1937 by the Electro-Metallurgical (30., Niagara Falls,New York.

